Not everyone plays with the same intensity*.
Based on the way that you've phrased the question, it seems like you as GMs have decided that unequal spotlight is a problem that the players have and the players aren't particularly enthused about the way you're going about solving it.
Are you sure that you're actually solving a problem the players really have? People play roleplaying games for all manner of reasons, and one of those reasons is to be entertained by the other players. People have different levels of skill, too, and some have a much easier time of fitting their character into an arbitrary scene than others do.
If you're just forcing people to do something they're not good at because of a problem only you can see, stop. It's not helping.
*...all the time.
It may also be possible you're expecting people to step up into a scene their aspects don't particularly care about. An easy way for measuring that is how many of their aspects might be compelled or invoked in the scene - if you can't think of one, maybe they just shouldn't be involved. If you can think of one, but only one, the case isn't that much better.
Aspects aren't the entirety of a character, of course, but they should at least be a significant approximation. And people have a lot more trouble finding something to do in a scene they don't really belong in.
Not all games need people to play with the same intensity all the time, either.
If you're playing a game that centers on precise tactical combat, everybody needs to be up for precise tactical combat every game session. If you're playing a game that centers on player-driven plotting with no definite sides, everybody needs to be up for self-directed scening every game session. If you're playing a game that centers on player appreciation of how adorable all the characters are, everybody needs to be up for cute, silly riffing on a central premise every game session.
Fate is an engine with a heavy bias toward proactive, capable characters that can take meaningful action right out the starting gate, but that doesn't make any game style particularly inevitable, and you have some more leeway to adapt Fate to your player group's preferences. It doesn't sound like they have a particular problem with taking turns in the formal structure of a conflict, so here are some ways you can put structure outside a combat:
A: No Unstructured Prompts
This is in keeping with a core axiom of Fate GMing about when to ask for rolls. Asking for a roll starting from the principle that the roll is needed for some parity in participation is likely to violate it.
Roll the dice when succeeding or failing at the action could each contribute something interesting to the game.
If you can't imagine an interesting outcome from both results, then don't call for that roll. If failure is the uninteresting option, just give the PCs what they want and call for a roll later, when you can think of an interesting failure. If success is the boring option, then see if you can turn your idea for failure into a compel instead, using that moment as an opportunity to funnel fate points to the players.
When to Roll Dice, Fate SRD, "What to Do During Play"
A solid way to set up something interesting happening is to present the PC you're prompting for action with some dramatic fact in the setting - a fleeting opportunity, or an imminent threat, along with enough setup that they understand what they stand to gain or lose.
B: The Advantage Two-Step
A simple way to set up someone else's participation in a course of action that seems likely to resolve things in one go is to turn one player making one roll into two players making two rolls; a create an advantage that's then followed up on. Because creating an advantage creates an aspect, which is a truth about the game world, setting this up is as easy as finding a reason why solving the problem in one go isn't possible.
As Dark Stobolous's Sable Troopers chase them through the cargo bay, Starhound declares a course of action and poses dramatically; he's going to shoot out the cargo bay release and dump all their pursuers into the blackness of space.
To set this up as a two-step, you could for example say that someone needs to set the troopers up so they'll be in position long enough for this trick to work, like Athens vaulting over them Creating an Advantage called She Went Thataway to snarl them up trying to pursue her, or that the Sable Troopers all have EVA gear and Starhound's shot will Create an Advantage on them called Outside Looking In that buys Twilliam some extra time to riffle through the cargo manifests to find the planet-cracker key they're looking for.
C: Other Things That Start With C
You could also spool things out into a different structured play element. Conflicts are only appropriate when each side wants to hurt the other, but even when your outspoken player isn't pointing that way, what they do can kick off a challenges, a contests, or a cliffhanger. (Cliffhangers were first seen in the pay-what-you-want supplement Masters of Umdaar. They're a challenge-like activity under dangerous circumstances with difficulties based on how you meet it, that can avoid the strict "don't repeat yourself" element of challenges that might be difficult to pull off in Accelerated.)
Transitioning to a different game element with its own rules can provide some structure to operate in, like requirements and goals, even if it doesn't have the strict measured nature of a conflict.